To meet the demand for higher integration density and operating speed of LSIs, the effort to reduce the pattern rule is in rapid progress. The wide-spreading flash memory market and the demand for increased storage capacities drive forward the miniaturization technology. As the advanced miniaturization technology, manufacturing of microelectronic devices at the 65-nm node by the ArF lithography has been implemented in a mass scale. Manufacturing of 45-nm node devices by the next generation ArF immersion lithography is approaching to the verge of high-volume application. The candidates for the next generation 32-nm node include ultra-high NA lens immersion lithography using a liquid having a higher refractive index than water in combination with a high refractive index lens and a high refractive index resist film, extreme ultraviolet (EUV) lithography of 13.5 nm wavelength, and double patterning version of the ArF lithography, on which active research efforts have been made. Also, resist materials sensitive to high-energy radiation of short wavelength, typically electron beam (EB) find use in the mask image writing application.
The ArF lithography started partial use from the fabrication of 130-nm node devices and became the main lithography since 90-nm node devices. Although lithography using F2 laser (157 nm) was initially thought promising as the next lithography for 45-nm node devices, its development was retarded by several problems. A highlight was suddenly placed on the ArF immersion lithography that introduces a liquid having a higher refractive index than air (e.g., water, ethylene glycol, glycerol) between the projection lens and the wafer, allowing the projection lens to be designed to a numerical aperture (NA) of 1.0 or higher and achieving a higher resolution. See Non-Patent Document 1.
In the photolithography using an ArF excimer laser (wavelength 193 nm) as the light source, a high sensitivity resist material capable of achieving a high resolution at a small dose of exposure is needed to prevent the degradation of precise and expensive optical system materials. Among several measures for providing high sensitivity resist material, the most common is to select each component which is highly transparent at the wavelength of 193 nm. For example, polymers of acrylic acid and derivatives thereof, norbornene-maleic anhydride alternating copolymers, polynorbornene, ring-opening metathesis polymerization (ROMP) polymers, and hydrogenated ROMP polymers have been proposed as the base resin. This choice is effective to some extent in that the transparency of a resin alone is increased.
Studies have also been made on photoacid generators. PAGs capable of generating perfluoroalkanesulfonic acids having a high acid strength are generally used in ArF chemically amplified resist compositions. These PAGs capable of generating perfluoroalkanesulfonic acids were already developed for use in the KrF resist compositions. For instance, Patent Documents 1 and 2 describe PAGs capable of generating perfluorohexanesulfonic acid, perfluorooctanesulfonic acid, perfluoro-4-ethylcyclohexanesulfonic acid, and perfluorobutanesulfonic acid.
Among these, perfluorooctanesulfonic acid and homologues thereof (collectively referred to as PFOS) are considered problematic with respect to their non-degradability and biological concentration in the environment. Manufacturers made efforts to develop partially fluorinated alkane sulfonic acids having a reduced degree of fluorine substitution as the replacement to PFOS. For instance, Patent Document 3 refers to the synthesis of α,α-difluoroalkanesulfonic acid salts from α,α-difluoroalkene and a sulfur compound and discloses a resist composition comprising a PAG which generates such sulfonic acid upon exposure, specifically di(4-tert-butyl-phenyl)iodonium 1,1-difluoro-2-(1-naphthyl)ethanesulfonate. Patent Document 4 refers to the development of α,α,β,β-tetrafluoroalkanesulfonic acid salts from α,α,β,β-tetrafluoro-α-iodoalkane and sulfur compound and discloses a resist composition comprising a PAG capable of generating such a sulfonic acid.
However, as the circuit line width is reduced by the recent rapid advance of technology, the degradation of contrast by acid diffusion becomes more serious for the resist material even when partially fluorinated alkanesulfonic acid generators as mentioned above are used. The reason is that the pattern feature size is approaching the diffusion length of acid, and this causes degradations of mask fidelity, line width roughness (LWR) and pattern rectangularity. Accordingly, to gain more benefits from a reduction of exposure light wavelength and an increase of lens NA, the resist material is required to increase a dissolution contrast or restrain acid diffusion, as compared with the prior art materials.
Under the circumstances, it was proposed to incorporate a PAG into a polymer for suppressing acid diffusion. For instance, Patent Document 5 describes a polymer using an acryloyloxyphenyldiphenylsulfonium salt as a monomer. Patent Document 6 proposes to incorporate the monomer into a polyhydroxystyrene resin for improving the LWR of this base resin. However, since the sulfonium salt is bound at its cation side to the polymer, the sulfonic acid generated therefrom upon exposure to high-energy radiation is equivalent to the sulfonic acids generated by conventional PAGs. These proposals are thus insufficient to suppress acid diffusion and unsatisfactory to overcome the outstanding problem. Also, Patent Document 7 discloses sulfonium salts having an anion side incorporated into the polymer backbone such as polystyrenesulfonic acid. The acids generated therefrom are arenesulfonic and alkylsulfonic acid derivatives which have too low an acid strength to sever acid labile groups, especially acid labile groups in ArF chemically amplified resist compositions. Patent Document 8 discloses polymers having three partially fluorinated sulfonic acid anions in combination with a specific lactone compound. Patent Document 9 describes exemplary three anions. Since they are esters of carboxylic acids which are strong acids, they are expected to be readily hydrolyzable and low stable. Copolymers derived therefrom have an insufficient solubility in resist solvents. Furthermore, Patent Document 10 discloses a sulfonium salt having a partially fluorinated alkane sulfonic acid anion as a polymerizable unit, which has insufficient resist performance in terms of resolution and LWR.
A resist material based on a polymer having PAG incorporated as polymerizable units, especially having an anion moiety incorporated in the polymer backbone is effective for suppressing acid diffusion because the acid-generating moiety is bound to the polymer backbone. The resist material, however, has the drawback that the resist film has a low transmittance and low resolution. This is because triphenylsulfonium is generally used as the counter cation.
Triphenylsulfonium salts are most often used as the PAG in the ArF lithography. They advantageously have good stability and acid generation efficiency in the resist film, but undesirably serve to reduce the transmittance and resolution of the resist film due to high absorption at the ArF exposure wavelength (193 nm). Aiming at a higher sensitivity and resolution, Patent Document 11 reports the development of alkylsulfonium salt form PAGs, specifically 4-alkoxynaphthyl-1-tetrahydrothiophenium cations and Patent Document 12 discloses a resist composition comprising a resin having a plurality of acid labile groups in combination with such salt. The acid diffusion suppression is still insufficient to gain acceptable mask fidelity and LWR.
There is a possibility that acid diffusion is suppressed without a substantial loss of transmittance by following the same concept as triphenylsulfonium salts, specifically by introducing a polymerizable functional group to a cation or anion moiety of an alkylsulfonium salt and incorporating the salt into a base polymer. However, in an attempt to synthesize an alkylsulfonium salt-containing base polymer, alkylsulfonium salt-containing units can be decomposed during polymerization, making it difficult to produce the desired polymer. This is because the alkylsulfonium salt is highly reactive with a nucleophile or the like and unstable. Patent Document 13 mentions to incorporate into a base polymer a sulfonium salt having a polymerizable functional group introduced in its anion moiety and lists up alkylsulfonium cations as typical counter cation, but describes nowhere actual synthesis examples.
Extreme ultraviolet (EUV) lithography is regarded promising as the exposure technology following the ArF lithography while electron beam (EB) lithography is adopted for mask image writing. For these lithography processes, it would be desirable to have a resist composition exhibiting improved properties including resolution, mask fidelity and LWR while minimizing the impact of acid diffusion so that the resist may comply with the current requirement of further pattern miniaturization.